A mobile wireless communications device may include a voltage regulated battery pack and mobile wireless communications circuitry coupled thereto. A typical voltage regulated battery pack and the mobile wireless communications device circuitry are generally not intrinsically safe (IS), and may be, for example, a spark source. A voltage regulated battery pack and mobile wireless communications device circuitry may be considered intrinsically safe if they collectively meet a standard for intrinsic safety, for example, if they meet the current Factory Mutual (FM) standard that defines IS requirements.
More particularly, a typical voltage regulated battery pack is not intrinsically safe and generally does not include an output that is infallibly voltage clamped to ensure that a maximum voltage used in determining intrinsic safety is not exceeded. For example, a typical battery pack mated with mobile wireless communications device circuitry may not meet the most current standard for intrinsic safety since the maximum voltage available from the battery pack is too high for a corresponding overall capacitance for the mobile wireless device circuitry and mated battery pack. For example, a battery pack that outputs 9.6V (1.6V maximum per cell×6 cells=9.6V) typically allows for a about 20-22 μF of total capacitance including the mobile wireless communications device circuitry. A capacitance above this level may make the mobile wireless communications device not intrinsically safe. Most mobile wireless communications device circuitry generally has a capacitance greater than what is allowed by the standard for the given output voltage of the voltage regulated battery pack.
One approach to making the mobile wireless communications device circuitry and the battery pack intrinsically safe includes reducing the capacitance of the mobile wireless communications device circuitry. However, reducing the overall capacitance of the mobile wireless communications device circuitry, for example, to the target 20-22 μF total capacitance value, may be below what is generally needed for the mobile wireless communication device to comply with operational guidelines, for example Federal Communications Commission (FCC) regulations, and to meet applicable performance standards, for example, Telecommunications Industry Association (TIA) standards.
Another approach includes selectively reducing capacitances within the mobile wireless communications device circuitry. For example, voltages may be clamped where relatively large capacitances are located in the mobile wireless communications device circuitry. Thus, the relatively large capacitances may be removed from the intrinsically safe analysis. However, such an approach generally requires additional discrete components, for example, two Zener diodes or silicon-controlled rectifiers (SCR), connected to the capacitors with a trace that should be 2 mm wide.
Other protection schemes for a battery pack may include circuitry to monitor the output voltage of the battery pack against a threshold. For example, U.S. Pat. No. 7,459,855 to Miyamoto discloses a battery voltage monitoring apparatus having a plurality of voltage sensors that output a signal when an over-voltage or low-voltage is detected, and a voltage clamp to limit the voltage. U.S. Pat. No. 4,709,202 to Koenck et al., U.S. Pat. No. 6,184,658 to Mori et al., and U.S. Pat. No. 5,804,894 to Leeson et al. all disclose battery pack monitoring circuits to provide voltage feedback and/or limit charging and discharging of the battery pack.
U.S. Pat. No. 6,518,731 to Thomas et al. discloses a protection circuit for use with a rechargeable battery pack and battery pack charger. The protection circuit includes a shunt regulator and temperature dependent resistor coupled in series between the charger and battery. The temperature dependent resistor is also coupled to the shunt regulator to limit current through the shunt regulator if the current exceeds a threshold.
U.S. Pat. No. 6,697,241 to Smith discloses a circuit for preventing high voltage damage metal-oxide-semiconductor field-effect transistor (MOSFET) switch.
The circuit includes a rechargeable battery cell, a parasitic inductor, and a clamp circuit coupled to the MOSFET to conductivity thereof.
In typical mobile wireless communications device circuitry, there are a relatively large number of locations where clamping may be desired. Physical space constraints may limit the addition of components, especially where a reduced physical size of the mobile wireless communications device is important, for example, in a portable application. More particularly, higher powered Zener diodes are generally too large to fit within the mobile wireless communications device circuitry, and if more than one capacitor is being clamped by the same Zener diodes, traces to all other capacitances should be 2 mm wide. Moreover, in some locations, the relatively large capacitances cannot be clamped.